The immobilization of transuranic elements (e.g., neptunium, plutonium, americium, etc.) in a stable matrix is useful primarily for safely containing waste materials in a solidified waste form, but is also attractive in other applications including in manufacturing inert matrix fuels for transmutation and in the production of targets for irradiation and particle physics research. Several ceramics have been assessed as matrix materials including oxides (e.g., ZrO2, MgO, Al2O3, MgAl2O4, Y3Al5O12, CeO2 and pyrochlores), nitrides (e.g., ZrN) and composites (e.g. glass-ceramic, ceramic-ceramic and ceramic-metal).
Pyrochlores are ternary metal oxides with the general formula A2B2O7. Rare earth pyrochlores are pyrochlores in which A denotes a rare earth cation and B typically represents a transition metal capable of octahedral coordination. Rare earth zirconium pyrochlores (e.g., gadolinium zirconate, Gd2Zr2O7), show promise for use as a durable storage matrix for immobilizing transuranic actinides due to their high-temperature stability, high corrosion resistance, and excellent radiation resistance.
One preferred method for forming pyrochlores is solution combustion synthesis (SCS). In the SCS process, liquid solutions containing suitable metal salts (typically metal nitrates) and a reducing agent as the fuel are prepared. The fuel for an SCS process is classified based on its chemical structure, i.e., the type of functional groups in the molecule (e.g., amino, hydroxyl, carboxyl). Beneficially, the use of liquid solutions in SCS allows for mixing of the reactants on the molecular level. Upon initiation of an SCS reaction, the fuel and oxygen formed during decomposition of the oxidizer react to provide suitable conditions for a rapid high-temperature, self-sustaining formation reaction.
The broad spectrum of compositions possible in the pyrochlore system allows a range of tri and tetra-valent transuranic elements to be incorporated into durable ceramic frameworks. As such, SCS would appear to be an excellent candidate for use in the synthesis of transuranic-doped pyrochlores. Unfortunately, it has proven difficult to induce crystallinity in as-formed zirconate pyrochlores and as a result, formation methods must include prolonged post-annealing treatments to induce crystallization. Moreover, when considering the doping of pyrochlores with transuranic elements, the incorporation of these elements into the ceramic is not a trivial matter. For instance, transuranic nitrate solutions may contain impurities that induce the formation of precipitates in the precursor solution. Nitric acid is also itself an oxidizer, and the introduction of transuranic dopants from nitrate solutions alters the targeted fuel to oxidizer ratio for a given reaction. As such, currently known SCS procedures for incorporating transuranic elements into a stable ceramic are both time and energy intensive and therefore quite costly.
What are needed in the art are methods for incorporating transuranic elements into rare earth zirconate pyrochlores. Rapid and low-cost methods for forming transuranic-doped zirconate pyrochlores that meet the minimum attractiveness criteria for special nuclear material (SNM) would be of great benefit.